Printed circuit boards are laminated structures typically comprising a generally rigid, non-conductive base or core with conductive circuitry, generally in the form of a plurality of copper lines, applied on one or both sides of the core. The copper lines interconnect holes which extend through the core for receiving pins of various electrical components. The holes are typically lined with a conductive coating to provide a site for electrical contact with the various electronic components which are plugged into the printed circuit board and to provide a conductive bridge between the two sides of the printed circuit board. The circuitry is covered and protected by a non-conductive coating which does not extend into the holes.
The conductive circuitry can be applied to the core by first bonding a sheet of copper foil to each side of the core and then applying a photosensitive material over the copper foil. Illumination of the photosensitive material with ultraviolet light in a desired pattern cures the illuminated material. The material that was not illuminated is then removed to expose the underlying copper foil. The exposed copper foil is then electroplated with copper to a desired thickness to generate the circuitry lines which are then plated with a solder mixture of tin and lead. The remaining photosensitive material is then removed and the unwanted copper foil is etched away, e.g., with an etch solution which dissolves copper but not the solder plate. The remaining copper circuitry lines are then covered with a protective coating.
During a standard electroplating process, the periphery of the printed circuit board, i.e., the portions of the printed circuit board near its outer edges, tends to be at a higher current density than the center of the printed circuit board. Hence, copper metal deposits more rapidly near the periphery of the printed circuit board than at the center. The result of this is that by the time the copper has deposited at the center of the circuit board to form a line of a desired thickness, the copper forming a line near the periphery has typically deposited to a thickness greater than the thickness of the photosensitive layer. When this happens, the width of the depositing copper lines increases as the copper deposit begins to grow laterally over the top of the photosensitive layer. The resulting copper circuitry lines near the periphery develop a cross-sectional configuration resembling a mushroom.
It is desirable in printed circuit boards to have the copper lines as close to each other as possible while assuring that no electrical contact between the lines occurs unless specifically desired. The reason for having lines closer together is to be able to locate more electronic components on a circuit board for a given amount of circuit board area. However, the increase in the width of the copper lines in the high current density areas during the electroplating process provides a limiting factor as to how close the copper lines can be.
At the present time a ten mil copper line width with a ten mil spacing between the lines is considered standard. By conventional techniques, a five mil line width with a five mil spacing is possible. There is, however, an ongoing need for a process more efficient than prior art processes for producing lines with a five mil width and also for a process that can efficiently produce lines having less than a five mil width and spacing.